The Third Generation Partnership Project (3GPP) has initiated the Long Term Evolution (LTE) program to bring new technology, new network architecture, new configurations and new applications and services to wireless networks in order to provide improved spectral efficiency and faster user experiences. FIG. 1 shows an overview of an Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (E-UTRAN) 100 in accordance with the prior art. As shown in FIG. 1, E-UTRAN 100 includes three eNodeBs (eNBs) 102, however, any number of eNBs may be included in E-UTRAN 100. The eNBs 102 are interconnected by an X2 interface 108. The eNBs 102 are also connected by an S1 interface 106 to the Evolved Packet Core (EPC) 104. The EPC 104 includes a Mobility Management Entity (MME) 108 and a Serving Gateway (S-GW) 110.
FIG. 2 shows an LTE user-plane protocol stack 200 in accordance with the prior art. The protocol stack 200 is located in a WTRU 210 and includes the packet data control protocol (PDCP) 202, the radio link control (RLC) 204, the medium access control (MAC) 206 and the physical layer (PHY) 208. The protocol stack 200 may also reside in an eNB (not shown).
FIG. 3 shows an LTE control plane protocol stack 300 in the WTRU 210 of FIG. 2. The control plane protocol stack 300 includes the non-access stratum (NAS) 302 and a radio resource control (RRC) 304. Also included are the PDCP 306, RLC 308 and MAC 310, which together form the layer 2 sublayer 312.
During a handover a WTRU 210 may transmit uplink (UL) system data units (SDUs) from the PDCP (206). If the WTRU 210 does not receive a notice that the SDUs were successfully delivered, the WTRU 210 may retransmit the SDUs. The WTRU 210 may use a PDCP status report, received from an eNB, to determine which uplink PDCP SDUs to retransmit. The WTRU 210 may also use the report to discard SDUs, if the SDUs were received or acknowledged by the status report.
During a handover, a target eNB may retransmit, to the WTRU 210, downlink PDCP SDUs that were forwarded by a source eNB. The WTRU 210 may transmit a PDCP status report to the target eNB. The target eNB may utilize the PDCP status report to determine which PDCP SDUs to retransmit to the WTRU 210. The target eNB may also discard downlink PDCP SDUs, if the SDUs were received or acknowledged by PDCP status report. Once the WTRU 210 receives the SDUs, the WTRU 210 may reorder the SDUs and eliminate duplicates. The WTRU 210 may then deactivate any functions based on a timer, such as a flush timer, for example. For signaling radio bearers (SRBs), the RLC 204 may indicate that handover has occurred and the PDCP 202 may reinitialize state variables such as a PDCP sequence number (SN) and a hyper frame number (HFN). The state variables may also be set to zero. All PDCP SDUs and PDCP PDUs that have been previously stored may be discarded.
In order to perform NAS level retransmission, the access stratum (AS) (not shown) may provide an indication of an NAS message transmission failure, if the failure occurred due to handover or cell selection.